This invention relates to a transducer for measuring temperature by means of thermally-induced resistance changes. More particularly, this invention relates to a transducer which primarily consists of a length of temperature-sensitive wire, but which also includes as a portion of the total resistance a deposited conductive film which is not wire.
It is well known that certain metals exhibit known and repeatable changes in resistance with temperature. This phenomenon has been put to practical use by creating temperature transducers which are coiled-up lengths of wire. Generally the coils are wound with fine gauge wire in order to obtain a practical resistance in a small space and to minimize the cost of sometimes expensive metals. Platinum is universally acknowledged as the optimum material for this use, as it possesses excellent stability and repeatability over a wide range of temperatures. Nickel, copper, and other less expensive metals are commonly used over relatively narrow temperature ranges. Many physical configurations presently exist for wirewound temperature transducers.
For optimum stability and repeatability, the wire should be pure, should be wound in a manner which minimizes or eliminates strain and stress on the wire and should be fully annealed after winding. Otherwise, repeated temperature cycling will induce stresses which will change the wire's resistance by both stretching and work-hardening it. Extended time at elevated temperatures will tend to anneal the wire, which also will change its resistance. Impurities will cause deviations from the known characteristics of the wire, and depending on the impurity, may cause the wire's resistance to change with time.
The manufacture of wirewound resistance transducers requires care and attention to detail in order to wind the coil to the correct resistance. For platinum, a resistance error of only 0.1% produces a measurement error of 0.25.degree. C. or more, depending on the temperature being measured. The resistance is affected by the length and diameter of the wire, by stretching during winding and by variations in the amount of resistance change seen during annealing. Since each of these effects individually may easily exceed 0.1%, it is difficult, and therefore expensive, to produce precision temperature transducers. Often, even though great care is taken, a substantial portion of the transducers produced may not meet their required tolerances and must be discarded. This further serves to increase costs.
Platinum resistance thermometers (PRTs) historically have provided precise, highly repeatable temperature measurement. In the standards laboratory interpolations made using strain-free, high purity, carefully handled wirewound PRTs define temperatures on the International Practical Temperature Scale between certain fixed freezing or boiling points in the range from -183.degree. C. to +631.degree. C. Industrial wirewound PRTs, which generally compromise the strain-free design to provide ruggedness and which sometimes are purposely "doped" with impurities to approximate the resistance-versus-temperature tables of DIN 43760 (1), nevertheless provide the best practical accuracy and repeatability for measuring temperatures from -200.degree. C. to +650.degree. C.
Methods exist to adjust the length (and therefore the resistance) of the wire. For instance, a small loop of wire may be left exposed so that after annealing, a short portion of the loop may be pinched off with a small welder to reduce the overall length of the wire. This overcomes the above-mentioned tolerance problems, but introduces problems of its own. A typical transducer for industrial and commercial use will have a resistance of 100 ohms at 0.degree. C. and will be made of platinum wire having a diameter around 0.001 inches. The overall length of the wire will be approximately 20 inches. After annealing, the wire will be exceedingly soft and delicate and, in addition, will be difficult to see without substantial magnification. The length of wire to be pinched off will be small; for example, a tolerance of .+-.0.1% (.+-.0.25.degree. C.) will correspond to an overall length adjustment tolerance of .+-.0.02 inches or, when pinching off a loop, will require that the loop length be adjusted in increments of 0.01 inches or less. Such adjustment typically will need to be made by hand under a microscope, then verified by remeasurement in a constant-temperature bath. After adjustment, of course, the exposed loop will need to be protected from damage and restricted such that it does not further short-circuit to itself.
Recently, several manufacturers have introduced resistance temperature transducers created by depositing conductive films, generally platinum, onto nonconductive substrates such as aluminum oxide. One example of such a device is given in U.S. Pat. No. 4,146,957 (Toenshoff). In this example, a serpentine pattern of platinum thick film paste is deposited on a ceramic substrate. As described in the patent, the paste is formulated using high purity platinum (99.9% pure or better) and contaminant-free frit (glass) in an organic binder. The deposition is made using silk-screen techniques, followed by firing at a high temperature (at least 1450.degree. C.). Other manufacturers are known to be depositing platinum films using thin-film techniques, in which the platinum is deposited by vapor deposition methods.
Resistance film transducers offer advantages in ruggedness and cost. The film, being bonded directly to the substrate, is inherently more rugged than a strain-free coil of wire. The film deposition is done using high volume automated or semiautomated techniques. Finally, although the film as deposited generally is not better than .+-.20% precision, it is easily and quickly trimmed to value using air abrasive, scribing or laser techniques.
Film transducers generally fall short of strain-free wirewound transducers in performance. Because the film is bonded directly to the substrate, it exhibits strain effects as the temperature changes. This not only affects the the resistance-versus-temperature characteristics but also produces permanent shifts in resistance after excursions to extended temperatures. The films generally lack the stability of pure, solid platinum wire.